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As you only have one mount point on the bottom of the fork legs, you'll need to see if you can get the legs of the lowrider and the MTN rack to line up. Then an M5 bolt through the two racks on each side and into the dropout should sort the bottom mounting. The top mounting looks a bit more of a challenge. What's under the electrical tape on the rack? and is there a hole in your fork crown?
Ha! Of course, that would help.